Microwave oven



Feb 15, 1966 H. DURAS ETAL MICROWAVE OVEN Filed Oct. 23

FIG.

INVENTORS HERBERT DURAS BY KARL H WALLENFELS WWW/111w,

V Fla/M3442)" ATTOR NEYS United States Patent cc 3,235,701 MICROWAVEOVEN Herbert Duras, Oberderdingen, Wurttemberg, and Karl H. Wallenfels,Freiburg, St. Georgen, Germany, assignors to Miwag Mikrowellen AktienGesellschaft, a corporation of the Swiss Confederation Filed Oct. 23,1962, Ser. No. 233,195 1 Claim. (Cl. 219-1055) This invention relates ingeneral to microwave ovens and more particularly to apparatus forimpedance matching a source of microwave energy to an oven enclosure.

A major problem inherent in the use of microwave ovens is that themismatching of the source of microwave power, which is usually amagnetron tube, and the load can result in damage or destruction to themicrowave generator, that is, could damage the magnetron tube generatingthe microwave energy. For example, should the microwave power be turnedon when no food is present in the oven, large amounts of energy would bereflected back towards the source. Moreover, as foods change stateduring heating, i.e., from frozen to liquid, a change in electricalimpedance of the food occurs. Thus, when a microwave oven is turned on,a frozen object presents a very low impedance to the source of energy,and as the food absorbs microwave energy and thaws, its impedanceincreases.

Another problem inherent in the use of microwave ovens is the fact thatwhen a microwave oven is energized, certain modes of oscillation are setup in the oven which vary with the type of load being heated and furtherwill set up different field patterns depending upon whether theenclosure is empty or loaded. Thus, objects placed in the oven may ormay not be heated, depending upon whether or not they are positioned ina region where there is sufiicient field intensity to heat the object.

The principal object of the present invention is to provide apparatusfor matching the impedance of a microwave oven enclosure to a source ofmicrowave energy. An advantage of the present invention is that shouldthe source of microwave energy be turned on when no load is present inthe oven enclosure, any mismatching which may occur between the sourceand the enclosure will not be sufiicient to destroy the tube generatingthe microwave energy.

The invention resides in a microwave oven, having a source of microwaveenergy, the oven having a resonant element coupled to the ovenenclosure. The resonant element is either made of or covered with energyabsorbing material. When the microwave power is turned on and no load ora very small load is present in the oven enclosure, the absorbingresonant element will absorb substantially more energy than when a loadis present. Further, when the microwave power is turned on and a load ispresent in the oven enclosure, the element acts as a simple couplingelement without exhibiting resonant properties, coupling energy from thesource "to the enclosure.

Other objects and many of the attendant advantages of the invention willbe readily appreciated as the invention becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 depicts a microwave oven in accordance with the invention; and

FIG. 2 depicts a top sectional view of the microwave oven shown in FIG.1.

3,235,701 Patented Feb. 15, 1966 Referring now to FIGS. 1 and 2, thereis shown a microwave oven 2 having a working enclosure portion 3 intowhich objects 4, such as foods, are placed. A source of microwave energyfeeds energy into a Y-shaped waveguide (not shown) which in turn couplesmicrowave energy to a pair of waveguides 6, 8. Energy in the waveguides6 and 8 is transferred to the oven 2 through 1 the coupling slots 12 and14, respectively, which are common to both the waveguides 6, 8 and therear wall 16 of the oven.

Spaced from the rear wall 16 of theoven by means of a plurality ofscrews 18 is a resonant element 22. The element 22 may be either made ofor covered with energy absorbing material. The element is formed of acenter portion 24 and a recessed peripheral portion 26. The edge 26 hasa plurality of output coupling slots 28 formed therein.

Microwave energy is radiated into the oven through the coupling slots 12and 14. When the enclosure is loaded, that is, contains food or otherobjects which absorb microwave energy, the impedances transformed at theoutput coupling slots 28 of the resonant element 22 are such that theslots 28 work as a simple coupling element without resonant propertiesand transfer microwave energy from the input coupling slots 12 and 14 Ito the enclosure 3. However, at no load conditions when there is noenergy absorbing material present in the enclosure 3, the coupling slots28, owing to the different field distribution of microwave energy, see adifferentimpedance. The impedance seen by the coupling slots.

at no load allows the element 22 to appear as a resonant element toincoming microwave energy and part of the 1 bodiments should be coveredwith a suitable energy ab-' sorbing material to ensure appropriateabsorption of microwave power.

Obviously, many modifications and variations of the present inventionare possible in the light of the foregoing teachings. It is to beunderstood, therefore, that the invention is not limited in itsapplication to the de tails of construction and arrangement of partsspecifically described or illustrated, and that within the scope of theappended claims, it may be practiced otherwise than as specificallydescribed or illustrated.

We claim:

In an oven of the type having a heating enclosure for confiningmicrowave energy, the enclosure supporting a predetermined wave energymode pattern when no load is present in the enclosure, a source ofmicrowave energy,

means for guiding microwave energy from the source to the enclosure, theenclosure having an aperture in one wall for admitting the microwaveenergy, the improvement comprising:

an energy absorbing element disposed in the heating enclosure adjacentto the admitting aperture, the energy absorbing element being spacedfrom the 3" 4 adfiii tting aperture and being corifi'g'ure'dftoresonate2,888,543 57 1959 Halagen'seh 1 219--10.55

when the bven is ope rating at the no-load condition. L 2,943,175 6/1960Guanella 219-1055 References Cited by the Examiner FOREIGN PATENTSUNITED STATES PATENTS 2/1960 2,704,802 3/1955 Blass et a1. 219-4055-RICHARD M.'WOOD,Prim a ry Examiner. 2,820,127 '1/1958 Argentoetal219-1055

